Radar system

ABSTRACT

The invention relates to a radar system for enhancing the reception of desired echoes by the elimination of background clutter. The system comprises a monopulse radar system provided with a quadrature error detector wherein the magnitude of the quadrature error signal is used to control the gain or sensitivity of the monopulse receiver.

United States Patent I llnentur Robert P. Nelson [56] References Cited 's p. mmzn STATES PATENTS J- 2 9 9 3.130.402 4/1964 Kuek.......,..,. 343/16MX 5- LP d 3: 2 3.283.322 ll/l966 Hovdaetal 1. 343/l6M W 3,392,387 7/1968 Kirkpatrick .1 343/171 (7 3| Assignce The Lnlted States of America as represented by the Secretary of the Navy Primary Tubbesmg Attorneys-Edgar J. Brow-er, Arthur L. Branning. T. O. "m Watson, Jr and Rv R. Anderson |54| RADAR SYSTEM scmmilrnnwmg ABSTRACT: The invention relates to a radar system for (SZI U.S. Cl. 1 1 1 1 1 4 v 1 1 v. 1 343/7 A, enhancing the reception of desired echoes by the elimination 343; lo M, 343/l7.l R of background clutter. The system comprises a monopulse [TH] Int. CL. 1 v v G015 9/22 radar system provided with a quadrature error detector {50! Field ol Search 343/7 R, 16 wherein the magnitude of the quadrature error signal is used M, l7.l to control the gain or sensitivity of the monopulse receiver I ',/T .?2 24 J0 J8 I VARIABLE x I L A IIXER PREAIP I ar'rsuunoa 82 DETECTOR H 2 II t s e 1 1732255571 IICROIAVE couunnoa i I Awunea 2 ,0 /r 1.9 i 2/ 2a n VARIABLE A IN uixtn meme ATTENUATOR i 82 H ocrscron ,7: 7i? as E QA CLUTTER GENERATOR 1 DETEC BATE ,GATED 2 TO sun I THRESHOLD 27 ,a/ 34 90 LF. PEAK TO PEAK 04 i muse DETECTOR ans 1 1.116 AND GENERATOR I srnercnen 32 :3: INSTANTANEOUS a A l Aec AMP THRESHOLD I QUADRATURE CHANNEL PROCESSOR PATENTEUAUGTDNA 3 599 205 sump UF 5 FIG. 20

FIG. 2b

(A)NARROW TARGET RETURNS (B)VERY WIDE TARGET RETURNS IN SUM AND DIFFERENCE PATTERN PHASE DELAY NOT AFFECTED BORESIGHT ANGLE OFF BORESIGHT PATTERN 4 3 2 l Ol-23-4 ANGLE OFF BORESIGHT IILILTILILILII BOTH PATTERNS HAVE EQUAL BY ANGLE OFF BORESIGHT mwzommmm mzka mm NORMALIZED AMPLITUDE AND PHASE RELATIONS SIGNALS DUE TO NARROW AND WIDE TARGETS AS SEEN BY A MONOPULSE RADAR NOTE= IDEALIZED RESPONSE CHARACTERISTICS OF MONOPULSE ANTENNA AND COMPARATOR 'PATENTEU AUG 1 0 m7;

SHEET 3 BF 5 TARGET NQI 83 CASE I SINGLE UNIT TARGET ON BORESIGHT I I -9 b sum E Z 1+ Bl DIFF j fi A= NOTE NO QA PRODUCED CASE 2 TARGET NO.|

SINGLE UNIT TARGET B OFF BORESIGHT 0- AI SUM 3 '7: i 2: Al 8' B| -v DIFF al A=A|B| NOTEI NO QA PRODUCED VECTOR RELATTONSHTP FOR A SINGLE POINT TARGET 2 Q 2 O N 0 O N N N J 2 i o (Q AB 8 o E0 To Mv O 0 69 E 0E9 B G L L G 2 I R A R L a +8 A T. A A 1 in u T T T Q8 9 A 9 2 B A B 2 T 2 2 5 7A TH aA a on m A S S l l m B B a *A +A a2 T AA rd F U h .l. E E H S 0 S s TI 2 E E E U GD GD R .A RN N M m U AA AA FC E I M T..- l. 0 2 00 ED N WT W AD Cl E T U MM MTA T] A ME 0.: C C WGS OAA NMC VECTOR RELATIONSHIPS FOR TWO EQUAL TARGETS SEPERATED IN ANGLE FIG. 5

TARGET O.|

TARGET NO. 2

PATENTEU AUG 1 0 Ian SHEET 5 BF 5 CASE 3 TWO TARGETS AT +2 AND 2 NOTE=QA LARGER THAN CASEI AND CASE 2 VECTOR RELATIONSHIPS FOR TWO EQUAL TARGETS WITH INCREASED ANGLE SEPARATION RADAR SYSTEM STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates to radar systems and, more particularly, to radar systems for the enhancing of desired echoes while suppressing background clutter.

Clutter may be described as echoes that result from scatter -on the surface of ground or water or any turbulent condition on such, surface. Clutter differs from noise voltages in that .noise voltages are random in nature whereas clutter echo is partially repeated from scan-to-scan on the radar scope. Sea

clutter is caused by scattering of reflected energy from turbulence on the sea surface.

Various methods and apparatus for suppressing clutter are -ltnown. One of the most widely known methods is that of the sitivity time controlmethod, however, is that signalsof equal.

or lesser magnitude than the clutter echo are also below the operating range of the less sensitive receiver and thus they are lost as wellas the clutter.

Operation orradarequipment in an adverse weather envirorime'nt, particularly those equipments which employ frequency bands of 5,000 Inc/second and higher, is plagued byechoes caused .by' bacltscatter of energy from water vapor and precipitation .These weather-ec hoes leadto confusion of the operator and Iowerthe probability .of recognizing and a acquiringdesired targets even though the desired targets may be separated from the :clutter in rangeQangle and/or magnitude. In vthe case of radar-lets employing automatic detection devices,-.targ etacquisition-is usually. based on receiving echoes exceeding a minimum magnitude. Unfortunately,=the minimum magnitude-is also-exceededby much of the clutter echo and a severe; false alarm problem results.

Another .well-known method of eliminating disturbances such as sea or .gro'und'clutter-is that of the moving target indicator or the MT] radar..ln this system, successive scans of the received energy are comparedsClutter presented on one scan is notnecessarily; presented in the same magnitude in a succeeding. scan; Therefore, the MTI system 'would not operate-:effectively-to eliminate rain echoes .or oth'erarapidly varying clutterin'thexradar system. v

The obvious solution. of employing lower frequencies is often-denied due .to constraints of size and desired angular accuracy. Similarly, circularzpolarization;techniques, while'offering a reasonable measure of subclutter visibility, are not always practical toemploy,=-and when-employed cause-a reduction in targetrecho which may limit the useful rangeof the radar. For optimumdetection of targets :in-clutter, it is, there fore, alsodesirableto-use other differences in the signature of targetandcluttee-inadditiontothe different depolarization effects of raindrops-and-morecomplex-targets.

SUMMARY-F THE INVENTION The :echo characteristics of targets havinga large extent in angleauch as weathenclutter differfrom those'whose angular extent fissmall. This characteristic is discernible by radar equipment-employing monopulse anglessensing antennas and having a sum channel and at least one angle difference channel. The signature difference lies in a decorrelation of the phase of the antenna difference'signals relative to the the antenna sum signals forextendeti targets.

This characteristic m'ay'be'exploite'd by quadrature channel processing which consists of determining the relative phase of the sum and diflerence signals or the magnitude of the quadrature component of the difference and of modifying the characteristics of the receiver and/or thresholding device to prevent alarms on signals which are identified as clutter. One possible means for mechanizing this technique is shown in the following disclosure. Note that this technique does not itself result in a subclutter visibility. It will, however, allow the receiver to operate at maximum usable sensitivity without false alarms'at all times. It is also fully compatible with other recognition techniques such as circular polarization and pulse length discrimination.

It is clear that the instant invention offers many improvements over the undesirable structure and operation of the prior art devices. Unlike the prior devices, the'present invention reliably enchances the reception of desired echoes by the elimination of background clutter, effecting this enhancement through the use of a monopulse radar system provided with a quadrature errordetecto'r. I

An object of the present invention is'the provision of a radar system.

Another object of the present invention is the provision of a radar system-which enhances the reception of desired echoes.

Still another object is the provision of a radar system which utilizes the quadrature error voltage for signal enhancement.

Yet another object is the provision of a radar system which utilizes the quadrature error voltage-to control receiver sensitivity. 4

Yet another object is the provision of a radar system in which the quadrature error voltage is not used for display.

Other objects, advantages and novelfeatures of the inven tion willbecome apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a block diagram of the-invention;

FIG. 2 shows the normalized amplitudeand phase relations in sum and difference signals due to narrow and wide targets as seen-by a monopulse radar;

'FIG.. 3 shows idealized response characteristics of monopulse antennaand comparator;

FIG. 4 shows the vector relationships of a single point target;

FIG. 5 shows the vector relationships for two equal targets separated-in angle; and

FIG. 6 shows the vector relationships for; two equal'targets with increased angle separation.

DESCRIPTION OF-THEPREF-ERRED EMBODIMENTS Referring now. to FIG. 1, there will be seen a pair of input terminals 10, each one of which is connected to one of the pair of antennas commonly used in monopulseradars The two input terminals 10 lead to a microwave comparator 11, the

is then fed through a sum log IF amplifierlZ to-asum detector I 24, and a sum IF limiter 26. The output of sum IF limiter 26'divides andffee'ds-as inputs to aninphasedifference :detector 25 and also a IF phase lag 27. Theoutputs from sum detector 24 and inphase difference detector- 25 re both usedas a dual input to a normalizer 30. 'Note that the normalizer or dif phase of ferential amplifier 30 is not essential to the proper functioning of the invention and that the sum signal 36 could have been obtained directly from the sum detector 24. The normalizer 30 was used in this particular embodiment of the invention, however, since the difference to sum signal 38 coming from it is modified by the target extent to change the sensitivity of the angle tracking loop of an associated radar set.

Looking back to the difference'channel there is shown that the signal from the difference preamplifier 1'7 feeds to a variable attenuator l9 and from there it goes to a difference log IF amplifier 21. From difference log amplifier 21 the output divides and goes to a quadrature phase difference detector 23 and also to the inphase difference detector 25. The output of 90 IF phase lag 27 also forms a second input to the quadralure difference phase detector 23. In order to form a feedback circuit, the output from quadrature difference phase detector 23 feeds to a peak-to-peak detector and stretcher 31 from whence it goes to an instantaneous AGC amplifier 32, whose output is divided between variable attenuator l8 and variable attenuator 19 as an automatic means of adjusting receiver gain inversely as a magnitude of received clutter echo or thermal mine. The output of peak-to-peak detector and stretcher 31 iivides and also forms an input to a quadrature difference :hreshold 33 whose output feeds a quadrature difference gate generator 34 from whence the signal goes as an input to a :lutter gate 35. The sum signal output from norrnalizer or dif- "erential amplifier 30 as appearing on terminal 37 also serves is a second input to clutter gate 35, the output of clutter gate 55 then appearing on terminal 36 will be the sum signal unless he magnitude of the output signal from the peak-to-peak de e'ctor and stretcher 31 exceeds the set value of the quadrature lifference threshold 33. This causes the quadrature difference gate generator 34 to generate a gate which when applied to he clutter gate 35 attenuates the sum signal appearing at 36 llld thereby reduces its effect further downstream in the radar ct and external to this processor. The output signal amplitude 'rom stretcher 31 will normally be held constant below the set alue of the quadrature difference threshold 33 by the action if the IAGC amplifier 32 and the variable attenuators l8 and [9. The gains of the sum and difference circuits are reduced in iroportion to the value of quadrature difference to sum .ignals performing the function of an automatic gain control, \GC, based on angular target extent. The action of the clutter gate 35 is to.prevent'large resultant sum signals from appearng at 36 caused by a sudden increase in clutter return too apid to be compensated by the IAGC amplifier loop.

Turning now to the theory and operation of the present inention, it will be noted that the echo characteristics of targets |aving a large extent in angle such as weather clutter differ rom those whose angle extent is small. The signature diference lies in adecorrelation of the phase of the monopulse lifference signals relative to the phase of the monopulse sum ignals for extended targets. This difference is shown graphially in FIGS. 24 AND 2b explained in the following pararaph.

A radar employing an ideal amplitude-monopulse angleensing antenna. i.e., one with two apertures having a common has: center and squinted defraction patterns, and looking at singlepoint source or target will produce sum and difference ectors which are in phase or separated by exactly 180'. The slative magnitude of the difference vector will be a measure f the angular displacement of the target from boresight. This base relationship is degraded by displacement of the phase enters due to antenna tolerances and by the vector addition f random noise generated 'in the radar to the desired signals 1 each channel. In the practical amplitude-monopulse radar :t with good signal-to-n'oise ratios (exceeding 14 db.) the omponent of difference errordisplaced 1:90 from the sum \gnal vector (the quadrature error) for this point target will e 'quite small compared to the magnitude of the sum signal, 16. 2a.

Multiple sources or targets at one range, which have a mea- Jrable extent in angle, can be shown by analysis to produce-a quadrature signal which increases with source separation or target extent. Extended or multiple targets can be simulated on an instantaneous basis by two targets whose angular position, signal phase and amplitude may be varied at will. It can further be shown, by examination of the two target case, that the magnitude of the quadrature error signal relative to" the sum signal is a measure of angular displacementbetween the targets, without regard to their nominal position in the pattern. In order to simulate the great number of scatterers in rain or widely dispersed clutter, the position and magnitude of the two sources must vary in a broadly statistical manner squinted will produce difference signals which vary in phase and amplitude with respect to thesumsignals in essentially a random fashion, as shown in FIG. 2b.

Let us now briefly review how a quadrature error is produced by two point targets in a monopulse radar beam. The response of a particular monopulse antenna and comparator is the vector sum and difference of echo signals observed by two squinted patterns (A and B) having a common FIG. 4 is the vector diagram of the response of the antenna and comparator of FIG. 3 to a single target 1 of unit magnitude at angles of zero and minus 2 with respect to the boresight. Notice that the difference signal for a single point target will be zero, in phase or at I out of phase with respect to the sum signal and that no quadrature error is produced in either of the two cases considered.

In FIG. 5, two targets 1 and 2 of unit magnitude are separated by a constant angle. They are considered as having a signal phase differing by Where the two targets are symmetrical about the boresight line only a quadrature error is produced. Where they are off center, the same quadrature error signal vector is produced, but, in addition, a normal difference error signal results. Where the target angular separa tion is increased as shown in FIG. 6, note that there is a corresponding increase in the magnitude of the quadrature error signal. As the monopulse antenna and comparator shown can only define signals into two sets, those producing a composite signal in channels A and B, respectively, any number of targets (or one target having large angular extent) may be represented, on an instantaneous basis, by only two targets.

Turning now to the implementation and performance of quadrature processing techniques it will be seen that this characteristic may be exploited by Quadrature Channel Processing" which consists of determining the relative phase of the sum and difference signals or the magnitude of the quadrature component of the difference signal, and of modifying the characteristics of the receiver and/or thresholding device to prevent alarms on signals which are identified as clutter. One possible means for mechanizing this technique is shown in the block diagram of FIG. I. The mechanization shown operates in the following manner.

Signals entering the antenna are assumed to be composed of desired target echoes and undesired clutter echoes. These are separated into sum and difference components by microwave comparator 1'1 and are amplified together with-receiver thermal noise by preamps l6 and 17 before passing through attenuators l8, l9 and log [F 21 and 22. The magnitude of the difference echoes both in phase and in quadrature with the sum signal are phase detected by detectors 23 and 25. The detected signals in the quadrature difference channel are also entirely due to angle-extended clutter'and thermal noise combined.

The signals in the sum and in phase difference channel contain essentially the same magnitude of clutter and thermal noise signal as the quadrature difference channel, but in addi tion contain signal due to the desired target echo. The magnitude of the quadrature difference channel signal is maintained at a chosen value by employing negative feedback to control the gain of the difference receiver through the operation of peak-topeak detector and stretcher 31 and AGC amplifier 32, feeding back to variable attenuators, 18 and 19. The same gain control signal is used to equalize the gain of both the sum and difference receivers.

The signal in the quadrature difference channel due to clutter is noiselike in character in that it varies randomly in phase and magnitude with respect to the sum channel signal. As such, it will periodically approach zero. In order to prevent undue increase in sum receiver gain when the quadrature errors pass through zero, the error peaks are stretched by stretcher 31 to limit the speed of gain recovery long enough for the clutter signal to decorrelate in range and to cycle through a roughly complete set of amplitude variations. In addition, an adjustable quadrature difference threshold and sum channel blanking circuit composed of threshold 33, gate generator 34 and clutter gate 35 is provided to prevent alarms due to inherent delay in reducing the gain of the sum receiver as the peaks of the clutter signal increase rapidly.

Adjustment of the feedback path gain by AGC amplifier 32 and quadrature difference threshold 33 permit selection of a particular constant false alarm ratio (CFAR) by the sum channel threshold detector, which should remain essentially constant for all combinations of weather clutter and thermal noise.

Since the sum and in phase difference channels contain desired target energy, in addition to the noise and clutter energy being held constant, the output of the sum channel as applied to differential amplifier 30 is essentially proportional to target energy and the output of the inphase differential channel is essentially proportional to the angular position of the target. Due to the operation of the quadrature differential threshold and the clutter gate which is generated when that threshold is exceeded, sum signals will be sufficiently attenuated to prevent further processing by the sum signal threshold circuitry when they are suspected of containing excessive clutter echo.

This mechanization provides an automatic means of adjusting receiver gain inversely as the magnitude of received clutter echo or thermal noise. It then permits optimum detection of target echoes which exceed the adjusted value. Targets which have larger echoes than clutter or which appear in gaps in clutter can thus be detected without confusion.

Quadrature Channel Processing is also compatible with other recognition techniques such as circular polarization and pulse length discrimination. It may also be used in conjunction with any STC characteristics to reduce the dynamic range of automatic gain control required.

As the grazing angle approaches as in the case of side lobe returns from the terrain directly below the aircraft, return will be regarded as clutter and should not result in false alarms even where the reflectivity of the terrain is very high.

Note that while this technique does not of itself result in a subclutter visibility, it will, however, allow the receiver to operate at maximum usable sensitivity without false alarms at all times.

Having given in detail the structure and operation of the present device, it is apparent that there is disclosed a monopulse radar system which materially enhances the reception of signals even under the most adverse conditions of clutter and thermal noise through the generation of a quadrature error signal which is then used to control the gain or sensitivity of the monopulse receiver.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings.

What I claim is:

l. A radar system for enhancing the reception of desired echoes comprising input means for receiving both desired and undesired signals;

a microwave comparator coupled to the input means for establishing sum and difference signals;

a sum signal receiver connected to the comparator for amplifying the sum signal;

a difierence signal receiver connected to the comparator for amplifying the difference signals;

a sum signal detector coupled to the sum signal receiver;

a quadrature difference phase detector connected to the sum signal receiver and to the difference receiver; and

a gate circuit coupled to the quadrature difference phase detector and to the sum signal detector.

2. The system of claim I wherein the sum signal receiver and the difference signal receiver contain mixer, preamp, variable attenuator and IF circuits.

3. The system of claim 2 further including a feedback circuit from the quadrature difference phase detector to both varia ble attenuators in both receivers.

4. The system of claim 3 further including quadrature difference gate generator connected to the quadrature difference phase detector and the gate circuit to control the operation of the operation of the gate circuit.

5. The system of claim 4 further including a quadrature difference threshold coupled between the quadrature difference phase detector and the gate generator for setting the operating point ofthe generator and therefore the gate circuit. 

1. A radar system for enhancing the reception of desired echoes comprising input means for receiving both desired and undesired signals; a microwave comparator coupled to the input means for establishing sum and difference signals; a sum signal receiver connected to the comparator for amplifying the sum signal; a difference signal receiver connected to the comparator for amplifying the difference signals; a sum signal detector coupled to the sum signal receiver; a quadrature difference phase detector connected to the sum signal receiver and to the difference receiver; and a gate circuit coupled to the quadrature difference phase detector and to the sum signal detector.
 2. The system of claim 1 wherein the sum signal receiver and the difference signal receiver contain mixer, preamp, variable attenuator and IF circuits.
 3. The system of claim 2 further including a feedback circuit from the quadrature difference phase detector to both variable attenuators in both receivers.
 4. The system of claim 3 further including a quadrature difference gate generator connected to the quadrature difference phase detector and the gate circuit to control the operation of the operation of the gate circuit.
 5. The system of claim 4 further including a quadrature difference threshold coupled between the quadrature difference phase detector and the gate generator for setting the operating point of the generator and therefore the gate circuit. 